feat(Wind): Forward extension host protocol messages to Mountain

Add ForwardToMountain function in Preload.ts to relay binary extension host protocol messages to Mountain for gRPC relay to Cocoon. Implements bidirectional messaging by listening for cocoon:extensionHostReply Tauri events and posting responses back to the workbench port.

Adds performance.mark() calls for handshake-complete and message flow tracing. This completes the IPC bridge enabling VS Code extension host bidirectional communication with the workbench via Mountain.

Refs: Vine protocol (Mountain ↔ Cocoon)

Author: NikolaRHristov

Source/Preload.ts

@@ -186,7 +186,6 @@ const ipcRenderer = {
 
 const ipcMessagePort = {
 	acquire: (responseChannel: string, nonce: string) => {
-
 		// Create an in-memory MessageChannel.
 		// port2 is posted to the window so acquirePort() (ipc.mp.ts) picks it up
 		// via its window 'message' listener (filters e.data === nonce && e.ports[0]).
@@ -200,19 +199,28 @@ const ipcMessagePort = {
 		port1.start();
 
 		let HandshakeComplete = false;
-
-		port1.onmessage = (Event: MessageEvent) => {
-			if (HandshakeComplete) {
-				// After handshake, silently drop extension-host protocol
-				// messages (activate, executeCommand, etc.) until
-				// a real Cocoon relay is wired.
-				return;
+		let MessageCount = 0;
+
+		const ForwardToMountain = (Data: ArrayBuffer | Uint8Array) => {
+			// Forward binary extension host protocol messages to Mountain
+			// for relay to Cocoon via gRPC. Mountain will handle decoding.
+			const Invoke =
+				(window as any).__TAURI__?.core?.invoke ??
+				(window as any).__TAURI__?.invoke;
+
+			if (typeof Invoke === "function") {
+				const Bytes =
+					Data instanceof Uint8Array
+						? Array.from(Data)
+						: Array.from(new Uint8Array(Data));
+				Invoke("MountainIPCInvoke", {
+					method: "cocoon:extensionHostMessage",
+					params: [{ data: Bytes, responseChannel }],
+				}).catch(() => {});
 			}
+		};
 
-			// The first large message from VS Code is the init data
-			// (JSON-encoded IExtensionHostInitData wrapped in VSBuffer).
-			// Any message with byteLength > 1 is init data; single-byte
-			// messages are control (Ready=2, Initialized=1, Terminate=3).
+		port1.onmessage = (Event: MessageEvent) => {
 			const Data = Event.data;
 			const Length =
 				Data instanceof ArrayBuffer
@@ -223,13 +231,58 @@ const ipcMessagePort = {
 							? Data.byteLength
 							: 0;
 
-			if (Length > 1) {
-				HandshakeComplete = true;
-				// MessageType.Initialized → byte 1
-				port1.postMessage(new Uint8Array([1]));
+			if (!HandshakeComplete) {
+				// The first large message from VS Code is the init data
+				// (JSON-encoded IExtensionHostInitData wrapped in VSBuffer).
+				// Any message with byteLength > 1 is init data; single-byte
+				// messages are control (Ready=2, Initialized=1, Terminate=3).
+				if (Length > 1) {
+					HandshakeComplete = true;
+					try {
+						performance.mark("land:exthost:handshake-complete");
+					} catch {}
+
+					// Forward init data to Mountain for Cocoon
+					ForwardToMountain(
+						Data instanceof Uint8Array
+							? Data
+							: new Uint8Array(Data),
+					);
+
+					// MessageType.Initialized → byte 1
+					port1.postMessage(new Uint8Array([1]));
+				}
+				return;
+			}
+
+			// Post-handshake: forward extension host protocol messages
+			MessageCount++;
+			try {
+				performance.mark(`land:exthost:message:${MessageCount}`, {
+					detail: { bytes: Length },
+				});
+			} catch {}
+
+			if (Length > 0) {
+				ForwardToMountain(
+					Data instanceof Uint8Array ? Data : new Uint8Array(Data),
+				);
 			}
 		};
 
+		// Listen for Cocoon → workbench messages via Tauri events
+		const TauriListen = (window as any).__TAURI__?.event?.listen;
+		if (typeof TauriListen === "function") {
+			TauriListen(
+				"cocoon:extensionHostReply",
+				(Event: { payload: { data: number[] } }) => {
+					if (Event?.payload?.data) {
+						port1.postMessage(new Uint8Array(Event.payload.data));
+					}
+				},
+			).catch(() => {});
+		}
+
 		// Send Ready after a tick so VS Code's onMessage listener is registered.
 		// MessageType.Ready → byte 2
 		setTimeout(() => {